1. Field of the Invention
This invention relates to close-range radio communications in general. More specifically, this invention relates to a radio system for data transmission by microwaves between a central station and a passive end that does not have its own UHF source and that operates in a response mode.
Such a radio system can be used to exchange information over a short distance between a main point or central station and one or more passive ends or terminal stations which should be very simply designed to meet cost and space requirements. This type of radio system has numerous applications, e.g. in:
remote-controlled data gathering systems such as a remote-controlled domestic counter reading system with a mobile radio accessor,
access or checking management systems with remote inquiry for code numbers memorized in individual access or checking cards,
road or rail signalling systems,
automatic sorting systems, e.g. for parcel post,
automatic invoicing systems for contact-free chip cards.
2. Description of the Prior Art
There are several known radio systems with a passive end in the prior art. Usually, an amplitude modulation is used to transmit data in the transmission direction going from the passive end towards the central station. An incident microwave transmitted by the central station is received by the passive end which modulates in amplitude by the data to be transmitted the incident microwave received or one of its harmonics before retransmitting it towards the central station. The passive end is supplied by the incident microwave. A rectifying and filtering circuit is usually provided to produce a d.c. voltage power supply from the received incident microwave.
The modulated microwave retransmitted by the passive end towards the central station has a low output due to the low energy output rate at reception of the incident microwave and to the energy deducted to supply the passive end, and consequently the range of such a radio system is necessarily limited. Moreover, the amplitude modulation has low immunity from noise compared to a frequency or phase modulation, which does not favour the range of the system embodying the prior art.
A solution to improve the range of a radio system with a passive end will be e.g. to replace the amplitude modulation of the retransmitted microwave by a two phase state modulation providing better immunity from noise. However, this solution is difficult to implement as the passive end should be very simple, thus excluding the possibility of using known phase modulators whose design is too complex.